1. Field of the Invention
The present invention relates to mobile communication technologies. More particularly, the present invention relates to an X2 interface setup method and a cell switching method in a mobile communication system.
2. Description of the Related Art
Along with recent developments of mobile communication technologies, a System Architecture Evolution (SAE) system has been widely used. More specifically, the SAE system includes an Evolved Universal Terrestrial Radio Access Network (E-UTRAN) and a core network.
FIG. 1 is a diagram illustrating an SAE system architecture according to the related art. A Base Station (BS) under the SAE system belongs to the E-UTRAN 110, and is responsible for providing User Equipment (UE) with a radio interface for accessing the SAE system. A Mobile Management Entity (MME) and a user plane entity (i.e., a Serving GateWay (S-GW)) 150 belong to the core network, and are connected with the BS under the SAE system through an S1 interface. Here, it is possible that the MME and the S-GW 150 are located in the same physical entity. FIG. 1 illustrates an example in which the MME and the S-GW 150 are located in the same physical entity, which is denoted as MME/S-GW. More specifically, the MME is responsible for managing a session context and/or a mobile context of the UE and saving security-related-information of a user, and the S-GW provides a user plane function.
Referring to FIG. 1, the BS under the SAE system includes a home BS 130 (e.g., a Home enhanced Node B (HeNB) or Home Node B (HNB)) and an ordinary macro BS 120 (e.g., enhanced Node B (eNB)). More specifically, the home BS 130 is used for a family and can be applied to places such as a university and/or a company, and is a plug-and-play type. That is, the HeNB is the home BS 130 applied to a long term evolution system and is connected with a gateway 140 of the home BS 130 through the S1 interface, and the gateway 140 of the home BS 130 is connected with the MME/S-GW 150 through the S1 interface. The HeNB may also be directly connected with the MME/S-GW 150 without the gateway of the BS. The HNB is the home BS 130 applied to a Universal Mobile Telecommunications System (UMTS) system and is connected with the gateway of the BS through an Iuh interface, and the gateway of the BS is connected with a Serving GPRS Support Node (SGSN) through the Iuh interface. Typically, the home BS 130 has a limit for an accessed UE, i.e., only the UE having an authority for accessing the home BS 130 can access the home BS 130, which is different from the ordinary macro BS 120. For example, regarding the user's family, only a member of the user's family or another member permitted by this family can access the home BS 130 of the family. Regarding a company, only a staff of the company and a cooperation partner permitted by this company can have access.
Typically, a group of home BSs having the same access user group (e.g., the home BSs used for the same company) is denoted as a Closed Subscriber Group (CSG). There is a unique IDentifier (ID) for each CSG, and the CSG ID is broadcasted in broadcast information of the home BS. In the related art, in order for the home BS to provide increased coverage, three access modes, which respectively are a Closed mode, a Hybrid mode and an Open mode, are defined for a cell of the home BS. The three access modes are respectively described in more detail below.
If the access mode of the cell of the home BS is the Closed mode, only a particular user, i.e., a user having a subscription with an operator, can access the cell of the home BS. A supported CSG ID is broadcasted in the cell of the home BS. Typically, the cell of this home BS in which the CSG ID is broadcasted and the access mode, which is the Closed mode, is denoted as a CSG cell. That is, the CSG cell denotes a closed-mode cell which supports a CSG function. If the access mode of the cell of the home BS is the Hybrid mode, the cell is denoted as a Hybrid cell. It is also required to broadcast the supported CSG ID in the Hybrid cell. Any of the users can access this Hybrid cell, but the member that belongs to the CSG supported by this Hybrid cell, i.e., the user already having a subscription with the operator, can gain a better service. That is, in the event that the Hybrid cell has a limited resource, firstly the member that does not belong to this home BS is switched to another cell, or is provided with a service using a low Quality of Service (QoS). If the access mode of the home BS is the Open mode, the home BS is the same as a macro cell, i.e., any user is permitted to have access. Also, no CSG ID is allocated to the cell of the home BS, and no CSG ID is broadcasted. The cell of the home BS of the Open mode is no different from the ordinary macro BS, and is denoted as an Open cell hereinafter.
Typically, a list of the CSG ID which permits access of the UE is saved on both the UE and the MME. This list may be subscription information. When the user accesses the cell of the home BS, the UE and the MME perform access control. That is, if it is determined that the access mode of the cell of the target home BS to be accessed by the user is the Closed mode, the user is permitted to have access in the event that the CSG supported by the cell of the home BS belongs to the list of the CSG ID which permits access. If the access mode of the cell of the home BS is the Hybrid mode, the user is permitted to have access and it is determined that the user is the member of the home BS in the event that the CSG supported by the cell of the home BS belongs to the list of the CSG ID which permits access, and the user is permitted to have access and it is determined that the user is not the member of the home BS otherwise.
Typically, a situation in which the UE moves from a home BS to another home BS, and particularly in which the UE moves from among multiple home BSs which belong to the same CSG, frequently occurs. With respect to this situation, handover based on the S1 interface (which is also denoted as a S1 handover) and handover based on an X2 interface (which is also denoted as an X2 handover) are the most common measures. More specifically, being compared with the X2 handover, the S1 handover has shortcomings such as frequent signaling interaction and/or core network requirements. A specific X2 handover procedure is as follows. When the UE of a connected mode moves from an eNB1 to another eNB2, if there is an X2 interface between the two eNBs, the eNB1 transmits a message to the eNB2 and the eNB2 prepares a resource. Thereafter, the eNB1 notifies a new configuration of a target cell to the UE, and the UE is synchronized with the target cell and transmits a message to notify the target cell. The BS in which the target cell is located, i.e., the eNB2, transmits a message to the MME after receiving the notification from the UE, so that the MME switches a downlink data tunnel from a source cell to the target cell.
It can be found that the X2 interface handover procedure in the related art is completely performed with respect to the macro BS eNB. The main reason for this is that in a system structure, which is illustrated in FIG. 1, there is an X2 interface between the macro BSs and there is no X2 interface between the home BSs. Therefore, the UE does not move between the home BSs.